This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. P2001-293921, filed on Sep. 26, 2001; the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a constant current circuit switching a output current.
2. Description of the Related Art
A constant current circuit outputting a constant current irrespective of an external output voltage is used in a driver of a light emitting diode (LED). For instance, the constant current circuit is installed in a device turning on and off an LED, or a control unit of a display device having an array of LEDs.
A current mirror circuit having no temperature dependency, and always outputting a stabile current, is used in a conventional constant current circuit. The current mirror circuit connects the base terminals of two transistors together, and further connects the base terminal to a collector terminal of one of the transistors together. A current flowing into one of the transistors is treated as a reference current, and the other transistor conducts a current equal to the reference current amplified by a constant magnification.
However, in the conventional constant current circuit, as the switching rate increases and/or the current decreases, there is a problem that output current duty varies according to the input capacitance of the base terminal of the transistor used for the current mirror circuit.
In a first aspect of the present invention, a constant current circuit comprises, a reference current generator outputting a reference current, a current mirror circuit outputting an amplification signal of the reference current, an output transistor outputting a constant current to the load based on an amplification signal of the current mirror circuit, a signal source outputting a pulse control signal, an auxiliary switching circuit having a switch terminal outputting a switch signal in response to the pulse control signal, and a discharge terminal outputting a discharge signal to an input side of the current mirror circuit when the switch signal is stopped; and a switch circuit turning off the output transistor when the switch signal is provided.
In a second aspect of the present invention, a semiconductor integrated circuit comprises a semiconductor chip, a reference voltage source integrated on the semiconductor chip and outputting a reference voltage, an amplifier circuit integrated on the semiconductor chip, and inputting the reference voltage and performing impedance conversion, a voltage-current converter integrated on the semiconductor chip, and outputting a reference current based on the reference voltage outputted from the amplifier circuit, a current mirror circuit integrated on the semiconductor chip, and amplifying the reference current and outputting the resulting current, an output transistor integrated on the semiconductor chip and outputting a constant current to the load based on an output signal of the current mirror circuit, a signal source integrated on the semiconductor chip and outputting a pulse control signal, an auxiliary switching circuit integrated on the semiconductor chip, and having a switch terminal outputting a switch signal in response to the pulse control signal, and a discharge terminal outputting a discharge signal to an input side of the current mirror circuit when the switch signal is stopped, and a switch circuit integrated on the semiconductor chip, and turning off the output transistor when the switch signal is provided.